In Escherichia coli and Salmonella typhimurium it has been shown that selenophosphate serves as the selenium donor for the conversion of seryl-tRNA to selenocysteyl-tRNA and for the synthesis of 2- selenouridine, a modified nucleoside present in tRNAs. Although selenocysteyl-tRNA also is formed in eukaryotes and is used for the specific insertion of selenocysteine into proteins, the precise mechanism of its biosynthesis from seryl-tRNA in these systems is not known. Since selenophosphate is extremely oxygen labile and difficult to identify in biological systems, we used an immunological approach to detect the possible presence of selenophosphate synthetase in mammalian tissues. Using antibodies elicited to E. coli selenophosphate synthetase, the enzyme was detected in extracts of rat brain, liver, kidney, and lung by immunoblotting. Especially high levels were detected in Methanococcus vannielii, a member of the domain Archeae, and the enzyme was partially purified from this source. It seems likely that the use of selenophosphate as selenium donor is widespread in biological systems. A cDNA, which seems to be a gene for selenophosphate synthetase, was obtained from mouse cDNA library. The gene has a TGA codon inside the open reading frame that codes for a selenocysteine. A mutant gene in which the TGA codon was changed to TGC for cysteine was constructed, and both wild type and cysteine mutant genes were transiently expressed in COS-1 cells in the presence of 75Se-selenite.